The enzymes adenosine deaminase (ADA, EC 3.5.4.4) and cytidine deaminase (CDA, EC 3.5.4.5) function to deaminate natural aminopurine and aminopyrimidine nucleosides, respectively, in human and other organisms. They may also convert active nucleoside-based drugs into inactive metabolites. For example, the purine nucleoside drug arabinosyladenine (fludarabine, ara-A) is deaminated by ADA; the resulting compound, with the parent amino group replaced with hydroxyl, is inactive as an antitumor agent compared to the parent compound. Similarly, the antileukemia drug arabinosylcytosine (cytarabine, ara-C) is metabolically degraded by CDA into inactive arabinosyluracil.
CDA is a component of the pyrimidine salvage pathway. It converts cytidine and deoxycytidine to uridine and deoxyuridine, respectively, by hydrolytic deamination (Arch. Biochem. Biophys. 1991, 290, 285-292; Methods Enzymol. 1978, 51, 401-407; Biochem. J. 1967, 104, 7P). It also deaminates a number of synthetic cytosine analogs which are clinically useful drugs, such as ara-C mentioned above (Cancer Chemother. Pharmacol. 1998, 42, 373-378; Cancer Res. 1989, 49, 3015-3019; Antiviral Chem. Chemother. 1990, 1, 255-262). Conversion of the cytosine compounds to the uridine derivatives usually confers loss of therapeutic activity or addition of side-effects. It has also been shown that cancers that acquire resistance to cytosine analog drugs often overexpress CDA (Leuk. Res. 1990, 14, 751-754). Leukemic cells expressing a high level of CDA can manifest resistance to cytosine antimetabolites and thereby limit the antineoplastic activity of such therapeutics (Biochem. Pharmacol. 1993, 45, 1857-1861). Inhibitors of CDA could therefore be useful adjuvants in combination chemotherapy.
Tetrahydrouridine (THU) has been known as an inhibitor of cytidine deaminase for a number of years.
Various reports have suggested that co-administration with THU increases the efficacy and oral activity of cytidine-based drugs. For example, THU has been shown to enhance the oral activity of anti-leukemic agent 5-azacytidine in L1210 leukemic mice (Cancer Chemotherapy Reports 1975, 59, 459-465). The combination of THU plus 5-azacytidine has also been studied in a baboon sickle cell anemia model (Am. J. Hematol. 1985, 18, 283-288), and in human patients with sickle cell anemia in combination with orally administered 5-azacytidine (Blood 1985, 66, 527-532).

THU has also been shown to enhance the oral efficacy of ara-C in L1210 leukemic mice (Cancer Research 1970, 30, 2166; Cancer Invest 1987, 5, (4), 293-9), and in tumor-bearing mice (Cancer Treat. Rep. 1977, 61, 1355-1364). The combination of intravenously-administered ara-C with intravenously-administered THU has been investigated in several clinical studies in humans (Cancer Treat. Rep. 1977, 61, 1347-1353; Cancer Treat. Rep. 1979, 63, 1245-1249; Cancer Res. 1988, 48, 1337-1342). In particular, combination studies in patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) have been performed (Leukemia 1991, 5, 991-998; Cancer Chemother. Pharmacol. 1993, 31, 481-484).
5-Aza-2′-deoxycytidine (decitabine) is an antineoplastic agent for the treatment of myelodysplastic syndrome (MDS), with potential utility for the treatment of AML and CML as well. Like the other cytidine-based drugs, its oral bioavailability and efficacy are limited by deactivation by CDA. THU has been shown to improve the potency of decitabine in a sickle cell disease model in baboons (Am. J. Hematol. 1985, 18, 283-288). In addition, another known CDA inhibitor, zebularine, has been shown to enhance the efficacy of decitabine in mice with L1210 leukemia (Anticancer Drugs 2005, 16, 301-308).
Gemcitabine, another cytidine-based antineoplastic drug, has also been studied in conjunction with CDA inhibitors (Biochem. Pharmacol. 1993, 45, 1857-1861). Co-administration with THU has been shown to alter the pharmacokinetics and bioavailability of gemcitabine in mice (Abstr. 1556, 2007 AACR Annual Meeting, Apr. 14-18, 2007, Los Angeles, Calif.; Clin. Cancer Res. 2008, 14, 3529-3535).
5-Fluoro-2′-deoxycytidine (fluorocytidine, FdCyd) is another cytidine-based anticancer drug which is an inhibitor of DNA methyltransferase. The modulation of its metabolism and pharmacokinetics by THU in mice has been studied (Clin Cancer Res., 2006, 12, 7483-7491; Cancer Chemother. Pharm. 2008, 62, 363-368).
The results of the aforementioned studies suggest that there is therapeutic utility in the administration of CDA inhibitors together with cytidine-based drugs such as ara-C, decitabine, 5-azacytidine and others. However, early CDA inhibitors such as THU suffer from drawbacks that include acid instability (J. Med. Chem. 1986, 29, 2351) and poor bioavailability (J. Clin. Pharmacol. 1978, 18, 259).
There is therefore an ongoing need for new, potent and therapeutically useful inhibitors of CDA.